There is a non-reciprocal circuit device such as an isolator, a circulator, etc. as one of transmitting and receiving circuit parts for cellular phones, automobile phones, etc. used in a microwave band and a UHF band. Generally, the isolator and the circulator have small insertion loss in a signal transmission direction and large loss in an opposite direction, so that they are used to prevent the breakage of amplifiers.
FIG. 19 shows one example of the conventional isolators. This isolator is constituted by upper and lower metal casings 1, 12 functioning as magnetic yokes, a permanent magnet 2, an assembly 20, flat capacitors 8, 9, 10, a dummy resistor 11, and a resin casing 7.
The assembly 20 is constituted by a thin conductor plate having such a shape that three central conductors 4, 5, 6 are radially projecting from a circular shield plate, and a garnet-type ferrite (ferrimagnetic body) 3 disposed on a circular portion of the thin conductor plate, with the three central conductors 4, 5, 6 folded onto the garnet-type ferrite 3 and integrally overlapping via insulating layers therebetween.
The resin casing 7 has a circular recess 13a for the assembly 20 at center, and recesses 13b, 13c, 13d for flat capacitors around the circular recess 13a. The bottom of each recess 13a, 13b, 13c, 13d is provided with a connecting electrode 14a. The connecting electrode 14a is integrally formed by a thin conductor plate 14 of about 0.1 mm in thickness, and exposed from the sidewalls of the resin casing 7 as external terminals 15a-15f. The external terminals 15a-15c and the external terminals 15d-15f are exposed symmetrically on opposing side surfaces. The resin casing 7 is provided with terminal electrodes 16a, 16b connected to the central conductors 4, 5, and the terminal electrodes 16a, 16b are connected to the external terminals 15a, 15d on the side surfaces. The external terminal 15a and the terminal electrode 16a are formed by one integral thin conductor plate separate from the connecting electrode 14a, and the external terminal 15d and the terminal electrode 16b are formed by the other integral thin conductor plate separate from the connecting electrode 14a. 
Each flat capacitor 8, 9, 10 is received in each recess 13b, 13c, 13d of the resin casing 7. Each flat capacitor 8, 9, 10 is constituted by electrodes formed on upper and lower surfaces of a flat dielectric substrate, and the lower electrode of each flat capacitor is connected by soldering to the connecting electrode 14a appearing in each bottom of the recesses 13b, 13c, 13d. Received in the recess 13d are the flat capacitor 10 and a dummy resistor 11, whose one electrode is connected by soldering to the connecting electrode 14a, and whose other electrode is connected to the central conductor 6.
The assembly 20 is disposed in the recess 13a of the resin casing 7. The circular shield plate of the central conductors 4, 5, 6 is connected to the connecting electrode 14a by soldering. With this structure the central conductors are grounded. One end of the central conductor 4 is connected to the upper electrode of the flat capacitor 8 and a terminal electrode 16a, and one end of the central conductor 5 is connected to the upper electrode of the flat capacitor 9 and a terminal electrode 16b. 
The resin casing 7 is disposed on the lower casing 12. The lower casing 12 has a shape complemental to that of a recess 21 in the bottom of the resin casing 7. The connecting electrode 14a exposed from the recess 21 is connected to the lower casing 12 by soldering, so that the resin casing 7 is made integral with the lower casing 12. The permanent magnet 2 for applying a DC magnetic field to the garnet-type ferrite 3 is fixed to an inner wall of the upper casing 1. With the upper casing 1 mounted to the lower casing 12, a surface-mountable isolator is obtained. Incidentally, ends of the central conductors can be connected to the capacitors and the terminal electrodes, for instance, by soldering or spot welding (for instance, see JP 10-135711 A). Also, a circulator can be obtained when the same terminal electrode as that for the other central conductor is used in place of the dummy resistor 11.
With demand for miniaturization, higher performance and lower prices increasingly mounting not only for isolators but also for mobile communications equipment, present objectives are miniaturization by the level of several hundreds microns, increase in performance by the level of one-tenth of dB, and further reduction of cost. Though widely used at present are 5-mm-square isolators, there is demand for further miniaturization. Thus, the inventors have been developing isolators with a target of providing 4-mm-square isolators (nearly 40% reduction of mounting area).
When an isolator comprising flat capacitors is further miniaturized, the flat capacitors 8, 9, 10 and/or the garnet-type ferrite 3 should be reduced in size. However, because the capacitance C of the flat capacitor is represented by C=εr·ε0·S/d, wherein εr is a dielectric constant of a dielectric body, ε0 is a dielectric constant of vacuum, S is the area of an electrode, and d is the thickness of a dielectric body, the reduction of the flat capacitor in a planar size makes it necessary to use a dielectric body having a large dielectric constant εr or make the dielectric body thinner, to obtain the same capacitance. However, a dielectric body having a large dielectric constant εr generally tends to have a large dielectric loss. Thus, the larger the dielectric loss, the larger the insertion loss of the resultant isolator. In addition, the reduction of a dielectric body in thickness results in decrease in the mechanical strength of a flat capacitor, making it likely that breakage, cracking, etc. occur at the time of assembling a non-reciprocal circuit device.
The miniaturization of a garnet-type ferrite is accompanied by the problem that a frequency band, in which the desired insertion loss is obtained, is narrowed. Further, the reduction of a garnet-type ferrite in diameter results in decrease in inductance obtained by the central conductors and the garnet-type ferrite. Therefore, the capacitance of flat capacitors should be increased to obtain a necessary operation frequency, resulting in increase in the size of the flat capacitors. In the conventional non-reciprocal circuit device, the reduction of a garnet-type ferrite is at most 2.2 mm in diameter, and the use of a smaller garnet-type ferrite than this size results not only in the deterioration of electric characteristics but also in the necessity of using larger flat capacitors. Accordingly, in the conventional structure in which flat capacitors are arranged around an assembly, it has been difficult to provide a small non-reciprocal circuit device with practically acceptable characteristics.
In the conventional isolator, flat capacitors 8, 9, 10 are arranged around the assembly 20 in a U-shaped pattern. The flat capacitor 10 connected in parallel to the dummy resistor 11 is disposed perpendicularly to a row of external terminals 15a-15f mounted onto the resin casing 7. Accordingly, when a bending force is applied to the non-reciprocal circuit device by the flexure of the circuit board, onto which the non-reciprocal circuit device is mounted, etc., the resin casing 7 is deformed with the external terminals 15a-15f as a fulcrum. As a result, the flat capacitor 10 connected in parallel to the dummy resistor is broken.
As another example, JP 10-303607 A discloses an isolator having a structure, in which erect capacitors are arranged in a resin casing such that the electrode surfaces of capacitors are substantially parallel to the center axis of a garnet-type ferrite, a garnet-type ferrite being disposed in a space encircled by the capacitors. However, such an isolator comprises a resin casing with a complicated structure, and it is thus difficult to handle the capacitors during assembling.